Preplated stamped small outline no-lead leadframes having etched profiles

Abstract

For a semiconductor integrated circuit (IC) leadframe, a base metal sheet ( 10 in FIG. 3 B) of a first metal, including copper, has substantially parallel first ( 11 ) and second ( 12 ) surfaces and an adherent layer of a second metal 13 a and 13 b ), including nickel, covering both surfaces. A first layer ( 14 ) of a third metal, including palladium, adherent to the second metal, is pre-plated on the first surface in a thickness suitable for bonding wire attachment. On the opposite surface, a second layer ( 15 ) of the third metal is adherent to the second metal on the second surface in a thickness suitable for parts attachment. Alternatively, a layer of a fourth metal, including tin, is used for parts attachment. The leadframe structure is then stamped from the sheet so that the base metal is exposed at the stamped edges ( 10 a in FIG. 3 A). Finally, the exposed base metal is preferentially chemically etched so that it is contoured ( 10 b ) for maximum adhesion and imbued with affinity to polymeric compounds.

I claim:  
     
         1 . A leadframe for use in the assembly of integrated circuit chips, comprising: 
 a base sheet of a first metal, said sheet having substantially parallel first and second surfaces;    an adherent layer of a second metal covering both surfaces;    a first layer of a third metal adherent to said second metal on said first surface and a second layer of said third metal adherent to said second metal on said second surface, said third metal bondable and solderable;    a structure stamped from said sheet so that the stamped edges extend between said first and second surfaces and said edges expose said base metal; and    said exposed base metal contoured for maximum adhesion and imbued with affinity to polymeric compounds.    
     
     
         2 . The leadframe according to  claim 1  wherein said base metal is selected from a group consisting of copper, copper alloy, brass, aluminum, iron-nickel alloy, or invar, and selected so that said selected base metal provides a higher differential chemical etch rate than said second metal.  
     
     
         3 . The leadframe according to  claim 1  wherein said base metal has a thickness between about 100 and 250 μm.  
     
     
         4 . The leadframe according to  claim 1  wherein said second metal is nickel, or a nickel alloy.  
     
     
         5 . The leadframe according to  claim 1  wherein said second metal layer has a thickness in the range from about 0.5 to 3.0 μm.  
     
     
         6 . The leadframe according to  claim 1  wherein said third metal is palladium or a palladium/gold stack.  
     
     
         7 . The leadframe according to  claim 1  wherein said first layer of said third metal has a thickness in the range from about 20 to 60 nm.  
     
     
         8 . The leadframe according to  claim 1  wherein said second layer of said third metal has a thickness in the range from about 60 to 180 nm.  
     
     
         9 . The leadframe according to  claim 1  wherein said exposed base metal contour curves inward, including approximately concave, in order to elongate the base metal surface available for adhesion to other materials, including polymeric compounds, and to provide a mechanical lock.  
     
     
         10 . The leadframe according to  claim 1  wherein said affinity is created by an oxide surface layer of said base metal.  
     
     
         11 . The leadframe according to  claim 1  wherein said exposed base metal is treated with a material enhancing adhesion to polymeric compounds including molding compounds.  
     
     
         12 . A leadframe for use in the assembly of integrated circuit chips, comprising: 
 a base sheet of a first metal, said sheet having substantially parallel first and second surfaces;    an adherent layer of a second metal covering both surfaces;    a layer of a third, bondable metal adherent to said second metal on said first surface, and a layer of a fourth, solderable metal adherent to said second metal on said second surface;    a structure stamped from said sheet so that the stamped edges extend between said first and second surfaces and said edges expose said base metal; and    said exposed base metal contoured for maximum adhesion and imbued with affinity to polymeric compounds.    
     
     
         13 . The leadframe according to  claim 12  wherein said fourth metal is selected from a group consisting of tin, tin alloys including tin/copper, tin/indium, tin/silver, tin/bismuth, tin/lead, and conductive adhesive compounds.  
     
     
         14 . The leadframe according to  claim 13  wherein said tin/copper alloy has between about 2% and 15% copper.  
     
     
         15 . The leadframe according to  claim 12  wherein said fourth metal has a reflow temperature compatible with wire bonding temperatures and molding temperatures.  
     
     
         16 . The leadframe according to  claim 12  wherein said fourth metal has a reflow temperature above semiconductor assembly temperatures, whereby it is operable to be dissolved into soldering media.  
     
     
         17 . The leadframe according to  claim 16  wherein said soldering media include solder pastes and solder waves.  
     
     
         18 . The leadframe according to  claim 12  wherein said fourth metal layer has a thickness in the range from about 3 to 25 μm.  
     
     
         19 . An integrated circuit (IC) device comprising: 
 a leadframe comprising a base metal sheet, including copper, of substantially parallel first and second surfaces and a stamped structure of an IC chip mount pad and a plurality of lead segments, each having a first end near said mount pad and a second end remote from said mount pad;    said leadframe having an adherent layer of a second metal, including nickel, on said first and second surfaces;    said leadframe further having an adherent layer of a third, bondable and solderable metal, including palladium, on said second metal on said first surface in a thickness suitable for bonding wire attachment;    said leadframe further having an adherent layer of said third metal on said second metal on said second surface in a thickness suitable for parts attachment;    said leadframe further having said base metal exposed at the edges of said stamped structure extending between said first and second surfaces, and said exposed base metal contoured for maximum adhesion and imbued with affinity to polymeric compounds;    an IC chip mounted to said mount pad;    bonding wires interconnecting said chip and said first ends of said lead segments; and    encapsulation material surrounding said chip, bonding wires and said first ends of said lead segments, while leaving said second ends of said lead segments exposed, whereby optimum adhesion to molding compounds is enabled.    
     
     
         20 . The device according to  claim 19  wherein said device has a package of the small outline no-lead or leadless type.  
     
     
         21 . The device according to  claim 19  wherein said bonding wires are selected from a group consisting of gold, copper, aluminum and alloys thereof.  
     
     
         22 . The device according to  claim 19  wherein the bonding wire contacts to said first ends of said lead segments comprise welds made by ball bonds, stitch bonds or wedge bonds.  
     
     
         23 . The device according to  claim 19  wherein said encapsulation material is selected from a group consisting of epoxy-based molding compounds suitable for adhesion to said leadframe materials.  
     
     
         24 . An integrated circuit (IC) device comprising: 
 a leadframe comprising a base metal sheet, including copper, of substantially parallel first and second surfaces and a stamped structure of an IC chip mount pad and a plurality of lead segments, each having a first end near said mount pad and a second end remote from said mount pad;    said leadframe having an adherent layer of a second metal, including nickel, on said first and second surfaces;    said leadframe further having an adherent layer of a third, bondable metal, including palladium, on said second metal on said first surface in a thickness suitable for bonding wire attachment;    said leadframe further having a layer of a fourth, solderable metal, including tin, adherent to said second metal on said second surface in a thickness suitable for parts attachment;    said leadframe further having said base metal exposed at the edges of said stamped structure, extending between said first and second surfaces, and said exposed metal contoured for maximum adhesion and imbued with affinity to polymeric compounds;    an IC chip mounted to said mount pad;    bonding wires interconnecting said chip and said first ends of said lead segments; and    encapsulation material surrounding said chip, bonding wires and said first ends of said lead segments, while leaving said second ends of said lead segments exposed, whereby optimum adhesion to molding compounds is enabled.    
     
     
         25 . A method for fabricating a leadframe structure, comprising the steps of: 
 selecting a continuous strip of sheet-like base metal, including copper, having first and second opposite surfaces;    plating a layer of a first metal, including nickel, on both said surfaces;    plating a layer of a second metal, including palladium, on said first surface in a thickness suitable for bonding wire attachment;    plating a layer of said second metal on said second surface in a thickness suitable for parts attachment;    stamping said structure from said plated sheet so that the stamped edges extend between said first and second surfaces, exposing said base metal; and    preferentially chemically etching said exposed base metal so that it obtains an approximately concave contour.    
     
     
         26 . A method for fabricating a leadframe structure, comprising the steps of: 
 selecting a continuous strip of sheet-like base metal, including copper, having first and second opposite surfaces;    plating a layer of a first metal, including nickel, on both said surfaces;    plating a layer of a second metal, including palladium, on said first surface in a thickness suitable for bonding wire attachment;    plating a layer of a third metal, including tin, on said second surface in a thickness suitable for parts attachment;    stamping said structure from said plated sheet so that the stamped edges extend between said first and second surfaces, exposing said base metal; and    preferentially chemically etching said exposed base metal so that it obtains an approximately concave contour.